Are you on NBN? You can do a reset by pushing a pin through the back side of your router to factory reset it. After you have done the factory reset, you should still be able to use the internet. Some simple setup should do too. I don't feel like you need to enter your username and password to get connected.
I also do not think your modem is supplied by TPG. There shouldn't be a big problem... I'll watch this thread and see if I can help you.